Piston thermal management in an opposed-piston engine

ABSTRACT

An opposed-piston engine includes pistons, each piston having an annular cavity in the piston&#39;s sidewall and positioned between its crown and ring grooves to block transfer of heat from the crown to the piston body.

PRIORITY

This application claims priority to U.S. provisional application forpatent 61/646,784, filed May 14, 2012.

A CROSS REFERENCE TO RELATED APPLICATIONS

This application contains subject matter related to that ofcommonly-owned U.S. patent application Ser. No. 13/066,589, filed Apr.18, 2011 for “Combustion Chamber Constructions for Opposed-PistonEngines” and commonly-owned U.S. patent application Ser. No. 13/136,955,filed Aug. 15, 2011 for “Piston Constructions for Opposed-PistonEngines”.

BACKGROUND

The field is internal combustion engines. Particularly the field isrelated to constructions for thermal management of pistons. In someaspects, the field includes internal combustion engines in which the endsurface of a piston crown is insulated from the ring area of the piston.In some other aspects, the field includes high compression dieselengines, particularly opposed-piston diesel engines.

During operation of an internal combustion engine, combustion of anair/fuel mixture occurs in cylinder space defined by the end surface onthe crown of at least one piston reciprocating in the cylinder. Forexample, in an opposed-piston engine, combustion occurs in the cylinderspace defined between the end surfaces on the crowns of two opposedpistons near respective top dead center positions in a cylinder. Theheat of air compressed between the end surfaces of the crowns causesfuel injected into the heated air to burn. The cylinder space where fuelcombusts is typically referred to as a “combustion chamber”.

In order to maximize the conversion of the energy released by combustioninto motion, it is desirable to prevent heat from being conducted awayfrom the combustion chamber through the piston. Reduction of heat lostthrough the piston increases the engine's operating efficiency.Typically, heat transfer through the piston is reduced or blocked byinsulating the piston crown from the body of the piston. However, it isalso the case that retention of the heat of combustion at the endsurface of the piston can cause thermal damage to the piston crown andnearby piston elements.

Piston thermal management is a continuing problem, especially given theever-increasing loads expected from modern internal combustion engines.In a typical piston, at least four areas are of concern for thermalmanagement: the piston crown, the ring grooves, the piston under-crown,and the piston/wristpin interface. The piston crown can be damaged byoxidation if its temperature rises above the oxidation temperature ofthe materials of which it is made. Mechanical failure of piston elementscan result from thermally-induced material changes. The rings and ringgrooves and the lands that border the ring grooves can suffer fromcarbon build-up caused by oil heated above the coking temperature. Aswith the ring grooves the under surface of the piston crown can alsosuffer from oil coking.

A recent study indicates that an opposed-piston engine two-stroke cycleengine exhibits increased thermal efficiency when compared with aconventional six-cylinder four-cycle engine. (Herold, R., Wahl, M.,Regner, G., Lemke, J. et al., “Thermodynamic Benefits of Opposed-PistonTwo-Stroke Engines,” SAE Technical Paper 2011-01-2216, 2011,doi:10.4271/2011-01-2216.) The opposed-piston engine achievesthermodynamic benefits by virtue of a combination of three effects:reduced heat transfer due to a more favorable combustion chamberarea/volume ratio, increased ratio of specific heats from leaneroperating conditions made possible by the two-stroke cycle, anddecreased combustion duration achievable at the fixed maximum pressurerise rate arising from the lower energy release density of thetwo-stroke engine. With two pistons per cylinder, an opposed-pistonengine can realize additional thermodynamic benefits with enhancedpiston thermal management.

SUMMARY

Enhanced thermal management of the pistons of an opposed-piston engineis realized by provision, in each piston of a pair of opposed pistons,of an annular cavity positioned between the end surface of the piston'scrown and the top ring groove of the piston. During engine operation,this cavity reduces the transfer of heat from the piston crown to thepiston body, while at the same time reducing or preventing thermaldamage to the rings and coking of lubricant in the ring grooves.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational perspective view of a piston of a pair ofpistons in which end surfaces of the pair of pistons are formed todefine a combustion chamber construction of an opposed-piston engine.

FIGS. 2-4 are side sectional drawings showing an operational sequence ofan opposed-piston engine including a pair of pistons according to FIG.1.

FIG. 5 is an elevational perspective view of one piston of a pair ofpistons of an opposed-piston engine.

FIG. 6 is a side sectional drawing of a first embodiment of the pistonof FIG. 5 showing an annular cavity positioned between the end surfaceof the piston's crown and the top ring groove of the piston.

FIG. 7 is a side sectional drawing of the piston of FIG. 6 when thepiston is rotated 90° on its axis.

FIG. 8 is a magnified view of a portion of the piston of FIG. 6.

FIG. 9A is an exploded side sectional drawing of a second embodiment ofthe piston of FIG. 5 showing a tubular part, or sleeve, that is receivedon the crown and seats in place around the piston's end surface so as toclose the annular cavity. FIG. 9B is a side sectional drawing of thesecond embodiment of the piston of FIG. 5, with the sleeve seated on thecrown. FIG. 9C is a magnified view of a portion of the piston of FIG.9B.

FIG. 10 is a magnified partial view of a third embodiment of the pistonof FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1-4 illustrate a combustion chamber construction defined bycomplementary end surface structures of opposed pistons disposed in aported cylinder of an opposed piston engine. The combustion chamberconstruction is bordered by squish surface areas. Identical generallysymmetrical bowls are formed in the end surfaces of the opposed pistons,and the pistons are rotationally oriented to place complementary curvedsurfaces of the bowls in opposition in order to maximize the squishsurface areas of the squish zone.

The end surface structure of each piston has a periphery surrounding abowl defining a concave surface. The concave surface includes a firstportion curving away from a plane containing the periphery surfacetoward the interior of the piston and a second portion curving away fromthe first portion and protruding outwardly in part from the plane. Aconvex surface opposite the bowl curves away from the periphery andprotrudes outwardly from the plane. The convex surface meets the secondportion of the concave surface to form a ridge therewith. Preferably,but not necessarily, the bowl has a semi-ellipsoidal shape. The endsurface structure is provided on both pistons and the pistons aredisposed in the bore of a ported cylinder with their end surfacesoriented to place complementary curved surfaces of the end surfacestructures in opposition in order to define a combustion chamber.Preferably, but not necessarily, the combustion chamber space definedbetween these two end surfaces is, or is very close to, an elongatedellipsoidal cylinder, providing a generally symmetrical geometry toreinforce and sustain the tumble motion. This combustion chamberstructure adds a tumble to the bulk motion of air in the combustionchamber, thereby increasing turbulence which enhances air/fuel mixing.

The structures of the piston end surfaces that define the combustionchamber are essentially identical to each other; accordingly, the piston280 shown in FIG. 1 represents both intake and exhaust pistons. Thepiston 280 has an end surface 282. A flat, radially-extending area 284centered on the longitudinal axis of the piston 280 defines a peripheryof the end surface 282. A bowl 286 is formed within the periphery. Thebowl 286 has a concave surface 288 with a first portion 290 curvinginwardly from a plane containing the flat circumferential area 284,toward the interior of the piston 280, and a second portion 292 curvingoutwardly from the interior of the piston through the plane. The endsurface 282 further includes a convex surface 295 within the peripherythat curves outwardly from the plane. The convex surface 295 meets thesecond portion 292 of the concave surface 288 to form a ridge 296 thatprotrudes outwardly from the end surface 282. At least one notch 294extends through the periphery into the bowl 286; preferably two alignednotches 294 are provided.

Referring now to FIG. 2-4, two pistons 280 having end surfaces shaped asper FIG. 1 are shown at or near respective bottom dead center (BDC)locations within a ported cylinder 220. The pistons are rotationallyoriented in the bore of the cylinder 220 so as to align the end surfacesin complement; that is to say, the concave surface portion 290 of onepiston 280 faces the convex surface 295 of the other piston. Charge airis forced through the intake port 224 into the cylinder, as exhaustproducts flow out of the cylinder through the exhaust port 226. Forpurposes of scavenging and air/fuel mixing, the charge air is caused toswirl as it passes through the intake port 224. As the pistons 280 movefrom BDC toward top dead center (TDC) locations as per FIG. 3, theintake and exhaust ports 224 and 226 close and the swirling charge airis increasingly compressed between the end surfaces 282. As the pistons280 approach TDC, compressed air flows from the peripheries of the endsurfaces into a combustion chamber having a cavity defined between theend surface bowls. At the same time, compressed charge air nearer thelongitudinal axis of the cylinder continues to swirl. As the pistons 280move through their respective TDC locations, the opposing concave-convexsurfaces 290, 295 mesh with one another to give the combustion chambercavity an elongated, generally ellipsoidal shape. Opposing pairs ofnotches 294 (see FIG. 1) in the end surfaces 282 define injection portsthat open into the combustion chamber at opposing pole positions of theellipsoidal shape.

As per FIG. 5, the piston end surface 282 is formed in a crown 281 atthe upper end of the piston 280. One or more ring grooves 302 are formedin the sidewall 283 of the body 285 of the piston 280, underneath theperiphery 284 of the end surface 282. Piston rings (not shown) areseated in the ring grooves when the piston is fully assembled. Acircumferential groove, recess, trench, or cavity 300, formed along acircumference of the piston sidewall 283, is positioned between the endsurface 282 and the top ring groove. The cavity 300 reduces or blockstransfer of heat from the crown through the lower part of the piston,functioning as a thermal resistor between the crown 281 and the ringgrooves 302. A thermally resistant material is disposed in the cavity300. Preferably, but not necessarily, the cavity 300 contains a materialwith low thermal conductivity. Examples of a low thermal conductivitymaterial include air, ceramics, and/or graphite. Preferably, the cavity300 is closed to form an annular chamber. For example, the cavity 300can be closed by a thin, flat encircling strip, or band, 305 that isseated in the mouth of the cavity and fixed to the piston structure.Closing the cavity 300 and/or filing the cavity 300 with ceramic,graphite, or other equivalent material adds structural integrity to thepiston. The cavity 300 provides a thermal resistance between the crown281 and the ring grooves 302 that reduces the transfer of heat into thepiston, thereby increasing the conversion of the combustion energy intomotion.

The thermal resistance of the cavity will cause the crown 281 to becomehotter, thereby increasing the possibility of oxidation. This can behandled in several ways. One is to manufacture the crown out of amaterial with a higher oxidizing temperature such as a stainless steelor nickel alloy. The other is to use standard piston materials and applya surface treatment which increases the surface oxidation temperature ofthe material. Material properties also degrade with temperature. Ifstandard piston materials are used, the piston can be designed with lowenough stress to still satisfy fatigue limits.

FIGS. 6-8 illustrate a first embodiment of the piston of FIG. 5. Theannular cavity 300 is positioned between the end surface of the piston'scrown 281 and the top ring groove 302 of the piston. Preferably, but notnecessarily, the cavity has the shape of a wedge in cross section, witha wide mouth that opens through the piston sidewall and tapers to aninner notch. Preferably, but not necessarily, the sidewall 283 and crown281 are formed as a single unitary piece by forging, casting and/ormachining. Alternately, the crown and piston body can be formed asseparate pieces that are joined by standard means such as welding,brazing, or threaded elements. If the cavity 300 is closed with the band305, the band is preferably made from a material that is thermallycompatible with the material of the piston structure to which it ismounted. For example, materials with equal, or substantially equal,coefficients of thermal expansion, are said to be “thermallycompatible.” Presuming the use of standard materials for the piston andthe band, the band 305 can be seated in the mouth of the cavity 300 andwelded in place. If the crown and piston body are formed as a unitarypiece, the band 305 can be fabricated with a gap 307 (best seen in FIG.5), thereby allowing it to be expanded slightly so as to fit around thecrown and moved downwardly therealong until seated in the mouth of thecavity 300. Once the band is seated, the gap 307 can be closed by thesame process by with which the band is welded to the piston body. Oncethe band 305 is seated and fixed in place by welding or an equivalentprocess, the two parts 300 and 305 cooperate to form a chamber that issubstantially airtight. The chamber can be filled with one or morematerials having low thermal conductivity. Alternatively, air can bedrawn from the cavity during the welding process such that the chambercontains a near-vacuum, that is to say, an annular space having apressure less than atmospheric pressure at sea level.

FIGS. 9A-9C illustrate a second embodiment of the piston of FIG. 5. Theannular cavity 300 is positioned between the end surface of the piston'scrown 281 and the top ring groove 302 of the piston. Preferably, but notnecessarily, the cavity has the shape of a wedge in cross section, witha wide mouth that opens through the piston sidewall and tapers to aninner notch. Preferably, but not necessarily, the sidewall 283 and crown281 are formed as a single unitary piece by forging, casting, and/ormachining. Alternately, the crown and piston body can be formed asseparate pieces that are joined by standard means such as welding,brazing, or threaded elements. The cavity 300 is closed with a sleeve325 having an upper flange 326 that extends in a radial direction and alower flange 327 that extends downwardly in an axial direction. Thesleeve 325 is assembled to an outer circumferential surface 328 thecrown 281, with the upper flange 326 held against a peripheral shoulder330 and the lower flange 327 seated in and covering the mouth of thecavity 300. As per FIGS. 9A-9C, the upper flange 326 of the sleeve 325constitutes the periphery 284 of the piston's end surface. The sleeve325 is preferably made from a material that is thermally compatible withthe material of the piston structure to which it is mounted. Presumingthe use of standard materials for the piston and the sleeve, the sleeve325 can be seated on the crown, with the lower flange seated in themouth of the cavity 300, and fixed in place by welding or an equivalentprocess. Once the sleeve 325 is seated and fixed in place, with the seambetween the lower flange 327 and the mouth of the cavity 300 closed bywelding or an equivalent process, the two parts 300 and 327 form achamber that is substantially airtight. The chamber can be filled withone or more materials having low thermal conductivity. Alternatively,air can be drawn from the cavity during the welding process such thatthe chamber contains a near-vacuum, that is to say, an annular spacehaving a pressure less than atmospheric pressure at sea level.

FIG. 10 illustrates a third embodiment of the piston of FIG. 5. In thisembodiment, the crown 281 and piston body 285 are formed separately,with oppositely-directed, axially-extending flanges 281 f and 285 f. Thecrown 281 and piston body 285 are brought together, with the pair offlanges 281 f, 285 f in alignment, while being welded along weld lines370 and 371. The cavity 300 thereby formed includes space on the insidesof the flanges 281 f, 285 f. If an electron beam welding process anear-vacuum can be created in the resulting chamber. Two useful massproduction methods include inertia welding and laser welding, either ofwhich can be used to join the crown and body pieces.

In some aspects, provision can be made to cool the interior of eachpiston by means of liquid coolant that circulates into the piston,across the backside of the crown, and out the bottom of the piston body.For example, with reference to FIG. 6, liquid coolant (lubricating oil,for example) flows into an annular gallery 256. The liquid coolantstrikes the interior surface of the annular gallery at its highest point260, thereby cooling that portion of the crown by impingement, and flowsfrom there throughout the annular gallery 256. From the annular gallery256, the liquid coolant flows into and through a central gallery 257.Liquid coolant flowing throughout the annular gallery 256 washes andcools an annular portion of the piston sidewall that includes ringgrooves 302. Liquid coolant flowing through the central gallery 257continuously irrigates an interior portion of the crown undersurface.See related application Ser. No. 13/066,589 in this regard.

Although a piston having a cavity to block transfer of heat from thecrown to the lower part of the piston has been described with referenceto presently preferred embodiments, it should be understood that variousmodifications can be made without departing from the spirit of thedescribed principles. For example, the piston can have a bowl of variousother shapes than as described and illustrated herein. Accordingly, theprinciples are limited only by the following claims.

The invention claimed is:
 1. An internal combustion engine including atleast one cylinder with longitudinally-separated exhaust and intakeports and a pair of pistons disposed in opposition to one another in abore of the cylinder, each piston including a piston body with a crownat one end, an end surface on the crown, in which an end surface of afirst piston has a bowl that cooperates with the end surface of anopposing piston to define a combustion chamber, at least one ring groovein the piston sidewall near the crown, an annular thermally-resistantcavity formed in the piston sidewall along a circumference of the pistonbody and positioned between the end surface and the at least one ringgroove, and one of a band, a flange, and a pair of opposing flangescovering the cavity to form a chamber that contains a near-vacuum. 2.The piston of claim 1, wherein the pair of opposing flanges comprises anupper flange that extends in a radial direction of the piston and alower flange that extends downwardly in an axial direction of thepiston, further wherein: a sleeve comprises the upper flange and lowerflange, and the sleeve is seated on an outer circumferential surface ofthe crown with the upper flange held against a shoulder of the crown andthe lower flange covering the cavity.
 3. The piston of claim 1, furtherincluding one or more cooling galleries within the piston body under thecrown.
 4. A piston for a two-stroke internal combustion engine,comprising: a piston body with a crown at one end; an end surface formedon the crown; the end surface including an elongated bowl thatcooperates with an opposing piston end surface to define a combustionchamber; at least one ring groove in the piston sidewall near the crown;an annular thermally-resistant cavity formed in the piston sidewall,extending along a circumference of the piston body, and positionedbetween the end surface and the at least one ring groove; and, one of aband, a flange, and a pair of opposing flanges covering the cavity toform a chamber that contains a near-vacuum.
 5. The piston of claim 4,wherein the pair of opposing flanges comprises an upper flange thatextends in a radial direction of the piston and a lower flange thatextends downwardly in an axial direction of the piston, further wherein:a sleeve comprises the upper flange and lower flange, and the sleeve isseated on an outer circumferential surface of the crown with the upperflange held against a shoulder of the crown and the lower flangecovering the cavity.
 6. The piston of claim 4, further including one ormore cooling galleries within the piston body under the crown.